mmc: renesas_sdhi: add eMMC HS400 mode support

This patch adds processing for selecting HS400 mode.

Signed-off-by: Masaharu Hayakawa <masaharu.hayakawa.ry@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>